1. Field of the Invention
The present invention relates to biodegradable plastic and a method for producing the same, and more particularly, to a polymer containing lactic acid as its constituting unit and a method for producing the same. Furthermore, the present invention relates to a method for producing a cyclic diester used for the production of the polymer. 2. Description of the Related Art
Nondegradable plastic waste causes environmental destruction. In recent years, biodegradable plastics have been extensively developed and studied. A biodegradable plastic may be decomposed by microorganisms, enzymes, or the like. Examples of biodegradable plastics include polyesters such as poly[(R)-3-hydoxybutylate] and polycaprolactone, and polyesters made of a glycol (e.g., ethylene glycol and 1,4-butanediol) and a carboxylic acid (e.g., succinic acid and adipic acid). However, since poly[(R)-3-hydroxybutylate] type polyesters are produced using a microorganism, they are expensive. Regarding the other polyesters, the use thereof is limited because of their low melting points.
Polylactic acid resin is used as a bioabsorbing material, such as a sutural thread for operations and a microcapsule for injections. Having superior biodegradability, the use of a polylactic resin for degradable plastic has been extensively developed. A polylactic resin has superior heat resistance and mechanical strength because of its high crystal melting point. Furthermore, after being decomposed, a polylactic resin hardly generates residues, ensuring the safety of any cleavage product.
However, polylactic acid has some disadvantages. That is, a polylactic acid is likely to be hydrolyzed and is unstable in water or an environment of high humidity. In addition, microorganisms slowly decompose polylactic acid.
Furthermore, polylactic acid has the following problems during melt molding. Polylactic acid becomes unstable when melted and decreases in molecular weight during melt molding. Thus, polylactic acid has poor molding stability and the physical properties of the resultant molding are decreased.
One of the reasons for instability of polylactic acid during melt molding is believed to result from a metal catalyst present in the polymer. In order to remove the metal catalyst, a method for inactivating the metal catalyst with a phosphorus compound, and a method for reprecipitating the polymer or washing the polymer with a solvent are known. However, according to the method for inactivating the metal catalyst with a phosphorus compound, the phosphorus compound is mixed with a molten polymer with high viscosity. Therefore, the efficiency is low. Furthermore, the phosphorus compound is harmful to the polymer and colors the polymer. According to the method for reprecipitating the polymer or washing the polymer with a solvent, complicated steps are required. Moreover, some of the solvent remains in the polymer.
In an attempt to solve the above-mentioned problems relative to polylactic acid, a copolymer containing lactic acid and a second component as its constituting unit has been developed (Polymer Bulletin, 25, 335, 1991, and Japanese Laid-Open Patent Publication No. 5-320323). However, copolymers simultaneously having stability during melt molding, hydrolysis resistance, and superior biodegradability have not been known.
A polyester of .alpha.-oxyacid, such as polylactic acid plastic, is generally synthesized by the ring opening polymerization of a diester, which is a cyclic dimer of .alpha.-oxyacid, by heating in the presence of a catalyst. This diester is usually prepared by polycondensing (dehydration condensation) .alpha.-oxyacid to give a precursor polymer, and depolymerizing the precursor polymer by heating. The diester thus prepared contains impurities inhibiting the ring opening polymerization, such as water, .alpha.-oxyacid, and an oligomer thereof. For this reason, a diester, which has been recrystallized several times, is used for the ring opening polymerization, thus increasing costs.
As an alternative method for preparing a diester, Japanese Laid-Open Patent Publication No. 63-165430 discloses a method for preparing a diester with high purity which includes the steps of: dissolving a diester in a hydrophobic organic solvent; extracting the diester with water in which a basic material is dissolved; and removing the organic solvent. However, this method is not sufficiently effective. Therefore, there has been a significant demand for a method for preparing an inexpensive diester.